This invention relates generally to mechanical systems having control systems. More particularly, the invention concerns control systems that adaptively compensate for harmonic distortion in mechanical systems that operate on a periodic basis.
Many mechanical systems experience position errors due to harmonic distortion. The harmonic distortion induces one or more components of the mechanical system to stray from its proper position thereby creating an amount of error. Control systems are utilized to compensate for the error by determining the error amount and indicating to a servomechanism to reposition by an amount equal to the error toward the proper position thereby tracking the proper position.
For many mechanical systems, the ability to track the proper location is vital to the system's operation. In such systems, harmonic distortion severely decreases the system's ability to function and, where error signals are involved, decreases the signal-to-noise ratio. For example, a magnetic recording disk used in a computer contains multiple concentric tracks used for data storage. A control system is used by the computer's disk drive to ensure that a data transducer reads/writes in the proper location, i.e. the center of a selected track on the disk. The disk is adapted to be rotated about an axis centered in disk's central spindle hole. If this hole was punched through the disk even slightly removed from the center axis of rotation, then the disk rotation will become eccentric causing the data transducer to vary from the center of a selected track creating a position error. The error will be a maximum amount once per rotation thus creating a once around error, or first harmonic, which will have to be tracked by the control system. Another example, a data recording medium having a base fabricated from an anisotropic material, such as MYLAR.RTM., is susceptible to changes in heat or humidity. Such changes would cause the disk to become elliptical creating a twice around error, or a second harmonic, which would have to be tracked by the control system. Further, if the disk was turned by an eight-pole motor then there may exist distortions in the eighth harmonic which would have to be tracked by the control system, in this case controlling the speed of rotation.
Another example of a mechanical system detrimentally affected by harmonic distortion is a drum printer. A focused light source in the drum printer projects light onto a receptive recording medium as the medium rotates within or on a drum, thus creating an image on the medium. If the drum is distorted in any way, the light source will have to refocus as the distance to the medium increases and decreases during the course of a revolution of the drum to avoid blurring or distorting the image. Distortions may occur in the drum if, for example, the drum was turned on a lathe utilizing a three-jaw chuck. The drum may then have distortions in a third harmonic. Or, a one-half harmonic may be created due to oil whip in a bearing as the drum rotates.
Conventional control theory teaches the use of a negative feedback loop to compensate for errors in a tracking system used, for example, in a disk drive. An example of such a loop is illustrated in FIG. 1. In such systems, a signal representing a measurement of error, as determined by the difference between current location and target or proper location, is transmitted back and its negated value is added into the tracking system to cause movement of the actuator by the amount of the error toward the target location. While responsive to the errors, this loop requires that the errors first occur before even attempting to correct them. This means that data from wrong tracks may be promulgated before any correction is attempted.
Another problem commonly found in current control systems is termed control system lag. Control system lag is the time delay between the time the tracking system detects an error and the time required for the control system to adjust to the proper location. Some of this delay is due to the electrical response of the control system, such as, for example, that resulting from a low sampling rate; the remaining delay is due to the mechanical response characteristics of the electromechanical actuator. These delays characterize the "bandwidth" of the control system. The greater the bandwidth the faster the positioning system can respond to an error condition thereby providing tightly controlled positioning. A positioning system having a high bandwidth is capable of providing increased data track density because tracks can be followed within a smaller tolerance.
Prior art teaches adding a memory loop to the aforementioned negative feedback loop so that the system can average data samples along a radial vector of a disk, for example, and predict that another point attained along the same radius will have the same error scaled accordingly. A problem with this method is that the memory loop needs to store a large number of samples per revolution and average them over many revolutions in order to achieve an acceptable noise suppression. Though this loop represents an improvement over the previously described loop, this loop at any given time examines only the samples along the same radius of the disk without any regard to the many other radii also contained in memory thereby creating a relatively low signal-to-noise ratio.
The previously described negative feedback loops are also susceptible to communicating incorrect error measurements when distortions become too great. This may occur when the central hole in the disk is severely decentered or the disk has distorted and become substantially elliptical, for example. The tracking system may cross a track and then use the incorrect track as the target, or proper, location and will calculate error with respect to the incorrect track instead of transmitting the substantial error that has actually occurred.
A prior art system, described in U.S. Pat. No. 4,536,809, entitled "Adaptive Misposition Correcting Method and Apparatus for Magnetic Disk Servo System" by Michael Sidman issued Aug. 20, 1985, employs a servo control system that periodically and iteratively determines position error. The harmonic contributions of the position error are passed through a filter having an impulse response that matches the characteristic waveform, a sinusoid.
A problem with the aforementioned approach is that the correction scheme adds the prior error with the current error before analysis. The effect is that error due to hardware is compensated for but the control system's ability to compensate for imprecision in the filter, representing the data model, is severely limited. This means that the filter applying the data model has to be near flawless and allows little margin for inaccuracy. Another problem is the storage of attained data as a vector. Using the vector requires that the entire vector must be constantly reprocessed through the filter using additional processing time. A third problem with the system is the fact that when it looks at the position error it is not able to look at read/write data and vice versa. And, since it only looks at the position error periodically, the runout may become substantial between reviews. Further, iterations to refine the harmonic contributions are expensive to the system in terms of data access speed for the same reasons. The system is also not readily adaptable to other control schemes due to the precision required in designing filters with accurate impulse responses.
Therefore, it is an object of the invention to decrease errors in control systems due to harmonic distortion thereby increasing data reliability.
It is another object of the invention to provide a control system predictive loop that is adaptive to changing conditions as well as easily adaptable to various control system designs.
It is yet another object of the invention to increase tracking reliability of a control system independent of the magnitude of the distortion.
These and other objects of the invention will be obvious and will appear hereinafter.